[EN] Background: Catalytic RNAs, or ribozymes, are regarded as fossils of a prebiotic RNA world that have remained in
the genomes of modern organisms. The simplest ribozymes are the small self-cleaving RNAs, like the ...[+]

[EN] Background: Catalytic RNAs, or ribozymes, are regarded as fossils of a prebiotic RNA world that have remained in
the genomes of modern organisms. The simplest ribozymes are the small self-cleaving RNAs, like the hammerhead
ribozyme, which have been historically considered biological oddities restricted to some RNA pathogens. Recent
data, however, indicate that small self-cleaving ribozymes are widespread in genomes, although their functions are
still unknown.
Results: We reveal that hammerhead ribozyme sequences in plant genomes form part of a new family of small
non-autonomous retrotransposons with hammerhead ribozymes, referred to as retrozymes. These elements contain
two long terminal repeats of approximately 350 bp, each harbouring a hammerhead ribozyme that delimitates a
variable region of 600–1000 bp with no coding capacity. Retrozymes are actively transcribed, which gives rise to
heterogeneous linear and circular RNAs that accumulate differentially depending on the tissue or developmental
stage of the plant. Genomic and transcriptomic retrozyme sequences are highly heterogeneous and share almost
no sequence homology among species except the hammerhead ribozyme motif and two small conserved domains
typical of Ty3-gypsy long terminal repeat retrotransposons. Moreover, we detected the presence of RNAs of both
retrozyme polarities, which suggests events of independent RNA-RNA rolling-circle replication and evolution,
similarly to that of infectious circular RNAs like viroids and viral satellite RNAs.
Conclusions: Our work reveals that circular RNAs with hammerhead ribozymes are frequently occurring molecules
in plant and, most likely, metazoan transcriptomes, which explains the ubiquity of these genomic ribozymes and
suggests a feasible source for the emergence of circular RNA plant pathogens[-]

Funding for this work was provided by the Ministerio de Economia y Competitividad of Spain (grants BFU2011-23398 and BFU2014-56094-P). Support of the publication fee was provided by the CSIC Open Access Publication Support ...[+]

Funding for this work was provided by the Ministerio de Economia y Competitividad of Spain (grants BFU2011-23398 and BFU2014-56094-P). Support of the publication fee was provided by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).[-]